An application programming interface (API) is a middleware that allows two applications to talk to each other through a representational state transfer (REST) channel. For instance, if you're developing a mobile application that uses a cloud database, you may create a PHP API to interface the Android client to a remote MySQL database. Another use-case of an API is to open your popular application to the end-users. Companies including Vultr, Google, Facebook, and Twitter have a public API. In thi......